The present invention relates generally to the field of modems, and, more particularly, to modem startup protocols.
The demand for remote access to information sources and data retrieval, as evidenced by the success of services such as the World Wide Web, is a driving force for high-speed network access technologies. Today""s telephone network offers standard voice services over a 4 kHz bandwidth. Traditional analog modem standards generally assume that both ends of a modem communication session have an analog connection to the public switched telephone network (PSTN). Because data signals are typically converted from digital to analog when transmitted towards the PSTN and then from analog to digital when received from the PSTN, data rates may be limited to 33.6 kbps as defined in the V.34 transmission recommendation developed by the International Telecommunications Union (ITU).
The need for an analog modem can be eliminated, however, by using the basic rate interface (BRI) of the Integrated Services Digital Network (ISDN). A BRI offers end-to-end digital connectivity at an aggregate data rate of 160 kbps, which is comprised of two 64 kbps B channels, a 16 kbps D channel, and a separate maintenance channel. ISDN offers comfortable data rates for Internet access, telecommuting, remote education services, and some forms of video conferencing. ISDN deployment, however, has been very slow due to the substantial investment required of network providers for new equipment. Because ISDN is not very pervasive in the PSTN, the network providers have typically tarriffed ISDN services at relatively high rates, which may be ultimately passed on to the ISDN subscribers. In addition to the high service costs, subscribers must generally purchase or lease network termination equipment to access the ISDN.
While most subscribers do not enjoy end-to-end digital connectivity through the PSTN, the PSTN is nevertheless mostly digital. Typically, the only analog portion of the PSTN is the phone line or local loop that connects a subscriber or client modem (e.g, an individual subscriber in a home, office, or hotel) to the telephone company""s central office (CO). In recent years, local telephone companies have been replacing portions of their original analog networks with digital switching equipment. Nevertheless, the connection between the home and the CO has been the slowest to change to digital as discussed in the foregoing with respect to ISDN BRI service. A recent data transmission recommendation issued by the ITU, known as V.90, takes advantage of the digital conversions that have been made in the PSTN. By viewing the PSTN as a digital network, V.90 technology is able to accelerate data downstream from the Internet or other information source to a subscriber""s computer at data rates of up to 56 kbps, even when the subscriber is connected to the PSTN via an analog local loop.
To understand how the V.90 recommendation achieves this higher data rate, it may be helpful to briefly review the operation of V.34 analog modems. V.34 modems are optimized for the situation where both ends of a communication session are connected to the PSTN by analog lines. Even though most of the PSTN is digital, V.34 modems treat the network as if it were entirely analog. Moreover, the V.34 recommendation assumes that both ends of the communication session suffer impairment due to quantization noise introduced by analog-to-digital converters. That is, the analog signals transmitted from the V.34 modems are sampled at 8000 times per second by a codec upon reaching the PSTN with each sample being represented or quantized by an eight-bit pulse code modulation (PCM) codeword. The codec uses 256, non-uniformly spaced, PCM quantization levels defined according to either the xcexc-law or A-law companding standard (ie., the ITU G.711 Recommendation).
Because the analog waveforms are continuous and the binary PCM codewords are discrete, the digits that are sent across the PSTN can only approximate the original analog waveform. The difference between the original analog waveform and the reconstructed quantized waveform is called quantization noise, which limits the modem data rate.
While quantization noise may limit a V.34 communication session to 33.6 kbps, it nevertheless affects only analog-to-digital conversions. The V.90 standard relies on the lack of analog-to-digital conversions outside of the conversion made at the subscriber""s modem to enable transmission at 56 kbps.
The general environment for which the V.90 standard was developed is depicted in FIG. 1. An Internet Service Provider (ISP) 22 is connected to a subscriber""s computer 24 via a V.90 digital server modem 26, through the PSTN 28 via digital trunks (e.g., T1, E1, or ISDN Primary Rate Interface (PRI) connections), through a central office switch 32, and finally through an analog loop to the client""s modem 34. The central office switch 32 is drawn outside of the PSTN 28 to better illustrate the connection of the subscriber""s computer 24 and modem 34 into the PSTN 28. It should be understood that the central office 32 is, in fact, a part of the PSTN 28. The operation of a communication session between the subscriber 24 and an ISP 22 is best described with reference to the more detailed block diagram of FIG. 2.
Transmission from the server modem 26 to the client modem 34 will be described first. The information to be transmitted is first encoded using only the 256 PCM codewords used by the digital switching and transmission equipment in the PSTN 28. These PCM codewords are transmitted towards the PSTN 28 by the PCM transmitter 36 where they are received by a network codec. The PCM data is then transmitted through the PSTN 28 until reaching the central office 32 to which the client modem 34 is connected. Before transmitting the PCM data to the client modem 34, the data is converted its current form as either xcexc-law or A-law companded PCM codewords to pulse amplitude modulated (PAM) voltages by the codec expander (digital-to-analog (D/A) converter) 38.
These PAM voltage levels are processed by a central office hybrid 42 where the unidirectional signal received from the codec expander 38 is transmitted towards the client modem 34 as part of a bidirectional signal. A second hybrid 44 at the subscriber""s analog telephone connection converts the bidirectional signal back into a pair of unidirectional signals. Finally, the analog signal from the hybrid 44 is converted into digital PAM samples by an analog-to-digital (A/D) converter 46, which are received and decoded by the PAM receiver 48. Note that for transmission to succeed effectively at 56 kbps, there must be only a single digital-to-analog conversion and subsequent analog-to-digital conversion between the server modem 26 and the client modem 34. Recall that analog-to-digital conversions in the PSTN 28 can introduce quantization noise, which may limit the data rate as discussed previously. The A/D converter 46 at the client modem 34, however, may have a higher resolution than the A/D converters used in the analog portion of the PSTN 28 (e.g., 16 bits versus 8 bits), which results in less quantization noise. Moreover, the PAM receiver 48 needs to be in synchronization with the 8 kHz network clock to properly decode the digital PAM samples.
Transmission from the client modem 34 to the server modem 26 follows the V.34 data transmission standard. That is, the client modem 34 includes a V.34 transmitter 52 and a D/A converter 54 that encode and modulate the digital data to be sent using techniques such as quadrature amplitude modulation (QAM). The hybrid 44 converts the unidirectional signal from the digital-to-analog converter 54 into a bidirectional signal that is transmitted to the central office 32. Once the signal is received at the central office 32, the central office hybrid 42 converts the bidirectional signal into a unidirectional signal that is provided to the central office codec. This unidirectional, analog signal is converted into either xcexc-law or A-law companded PCM codewords by the codec compressor (A/D converter) 56, which are then transmitted through the PSTN 28 until reaching the server modem 26. The server modem 26 includes a conventional V.34 receiver 58 for demodulating and decoding the data sent by the V.34 transmitter 52 in the client modem 34. Thus, data is transferred from the client modem 34 to the server modem 26 at data rates of up to 33.6 kbps as provided for in the V.34 standard.
The V.90 standard only offers increased data rates (e.g., data rates up to 56 kbps) in the downstream direction from a server to a subscriber or client. Upstream communication still takes place at conventional data rates as provided for in the V.34 standard. Nevertheless, this asymmetry is particularly well suited for Internet access. For example, when accessing the Internet, high bandwidth is most useful when downloading large text, video, and audio files to a subscriber""s computer. Using V.90, these data transfers can be made at up to 56 kbps. On the other hand, traffic flow from the subscriber to an ISP consists of mainly keystroke and mouse commands, which are readily handled by the conventional rates provided by the V.34 standard.
The V.90 standard, therefore, provides a framework for transmitting data at rates up to 56 kbps provided the network is capable of supporting the higher rates. The most notable requirement is that there can be at most one digital-to-analog conversion and no analog-to-digital conversion in the downstream path within the network. Nevertheless, other digital impairments, such as robbed bit signaling (RBS) and digital mapping through PADs which results in attenuated signals, can also inhibit transmission at V.90 rates. Communication channels exhibiting non-linear frequency response characteristics are yet another impediment to transmission at the V.90 rates. Moreover, these other factors may limit conventional V.90 performance to less than the 56 kbps theoretical data rate.
Articles such as Humblet et al., xe2x80x9cThe Information Driveway,xe2x80x9d IEEE Communications Magazine, December 1996, pp. 64-68, Kalet et al., xe2x80x9cThe Capacity of PCM Voiceband Channels,xe2x80x9d IEEE International Conference on Communications ""93, May 23-26, 1993, Geneva, Switzerland, pp. 507-511, Fischer et al., xe2x80x9cSignal Mapping for PCM Modems,xe2x80x9d V-pcm Rapporteur Meeting, Sunriver, Oreg., USA, Sep. 4-12, 1997, and Proakis, xe2x80x9cDigital Signaling Over a Channel with Intersymbol Interference,xe2x80x9d Digital Communications, McGraw-Hill Book Company, 1983, pp. 373, 381, provide general background information on digital communication systems.
While the V.90 standard (recommendation), like V.34, defines various requirements allowing modems from a variety of manufacturers to communicate, there is still variability between different modem designs as provided by different modem manufacturers. Some of the variations in modem design are specifically provided for by the V.90 standard. For example, bits 12 through 20 of the INFO0 signal are specified as flags characterizing various modem support capabilities as described in Table 7 of the V.90 standard. Other differences between modem designs exist beyond those specified by the standard. These differences can impact the data rate which may be obtained when modems of different types are connected both during the startup procedures and during the subsequent communication session. The impact of these differences may also vary depending upon choices made in the V.90 startup procedures. However, the V.90 protocol does not specify a requirement for communication of the manufacturer (i.e., design type) of a modem during startup procedures. While it is known in other arts, for example, with cable modems, to provide a unique identifier for a cable modem, this identifier is typically associated with a particular cable modem unit not a design type of a cable modem. In addition, U.S. Pat. No. 5,317,594 proposed a modification to a startup protocol to include transmission of a predetermined signal to allow modems conforming with the V.fast standard to recognize that they are communicating with another modem conforming to the V.fast standard. However, this is not an identification of a design type of a modem but rather specifies a supported protocol. U.S. Pat. No. 5,311,578 proposed inclusion of a low level identification signal as a tone hidden within a CCITT V.25 answer tone to allow use of a non-standard handshaking procedure. However, this typically requires both the call and answer modem to be configured to transmit and receive the identification signal tone, for example, with a tone detector.
Accordingly, variations in design type of modems on any given communication session may still result in a less than optimal connection being established during startup. Accordingly, there exists a need for improvements in modem technology to allow modems, particularly modems such as V.90 modems, to achieve more closely their theoretical maximum data rate.
It is an object of the present invention to provide modems which may be able to identify the type of a connected modem during startup procedures.
It is a further object of the present invention to provide modems which may utilize knowledge of the type of a connected modem to improve or optimize the configuration of a communication connection.
These and other objects, advantages, and features of the present invention are provided by methods, systems and computer program products for detecting whether a remote modem is of a particular design type and methods, systems and computer program products for adjusting the communication configuration for a communication session based on a type of the remote modem. The modem type is recognized based on a knowledge that the modem design of certain manufacturers has a unique associated data pattern contained within the startup communication sequence which is not specified by the protocol but which can be demodulated and detected to thereby recognize that the remote modem is a particular manufacturer""s design type. Based on knowledge of the characteristics of the remote modem design, various steps are taken which may improve performance of the communication connection in light of the particular modem design""s characteristics. For example, a different Total Harmonic Distortion (THD) threshold may be used for falling back to V.34 communications when the remote modem design only supports up to 2 bit look ahead for spectrum shaping.
In one embodiment of the present invention, a method is provided for detecting at a local modem whether a remote modem is a first type of modem, the first type of modem having a unique associated modem type data. A received signal from the remote modem is demodulated to provide data associated with the received signal. The data associated with the received signal is then compared with the unique associated modem type data to determine whether the remote modem is a first type of modem. In a particular embodiment, the local modem and the remote modem are at least one of a V.34 or a V.90 modem and the received signal is a phase 2 startup signal. More particularly, the received signal may be a fill bit field of INFO1 and the number of fill bits associated with the fill bit field may be provided as the data associated with the received signal. The remote modem is determined to be a first type of modem based on the number of fill bits associated with the fill bit field. More particularly, the fill bit field in one embodiment precedes a frame synchronization field of INFO1 and the number of fill bits indicating a first type of modem is thirteen.
In a further aspect of the present invention, a method is provided for adjusting a communication configuration for a communication session based on a protocol having an associated set of constellation points with a remote modem based on a type of the remote modem. The adjustment may be selected from various design type specific communication configuration options intended to improve performance of the modem for the communication session. The Total Harmonic Distortion (THD) threshold for falling back to V.34 communications may be set based on a type of the remote modem when the remote modem is a type of modem which supports only up to two bit look ahead for spectrum shaping. Selected constellation points of the associated set of constellation points may be excluded when the remote modem is a type of modem which modifies at least one of the selected constellation points in a manner not specified by the protocol. The use of longer than normal duration Digital Impairment Learning (DIL) sequences for high power mode may be disabled when the remote modem is a type of modem which has an associated retrain timer period shorter than a timer period specified by the protocol. Finally, an adaptation step size for use in equalizer convergence operations may be selected based on a type of the remote modem when the remote modem is a type of modem which provides a curve fitting error at a specified symbol rate that corresponds to a length of a local loop supporting the communication session.
More particularly, the protocol in a preferred embodiment in V.90 standard protocol. A lower THD threshold may be set when the remote modem is a type of modem which only supports up to 2 bit look ahead for spectrum shaping. The elected constellation points may be excluded based on a detected digital pad. In one embodiment, Ucode 86 and Ucode 102 constellation points are excluded if the detected digital pad is between about 2.7 dB and about 3.5 dB and Ucode 95 constellation points are excluded if the detected digital pad is between about 6.00 dB and about 6.02 dB. Constellation points in a further embodiment are excluded by setting learned levels associated with the excluded selected constellation points to about zero (and preferably zero) in a learned DIL sequence. In another embodiment, the specified symbol rate is 3429 and a larger adaptation step size is selected if the curve fitting error is below a predetermined threshold criterion.
As will further be appreciated by those of skill in the art, while described above primarily with reference to method aspects, the present invention may be embodied as methods, apparatus/systems and/or computer program products.